1. Technical Field
The invention relates to a method and to an apparatus for image acquisition and display by means of Nuclear Magnetic Resonance imaging (MRI) allowing the display of images of an anatomical region along section planes or slices having different positions and orientations one with respect to the other.
2. Discussion of Related Art
In the following description and in the claims, the terms “section plane” and “slice” have to be considered as having equivalent meanings. A section plane is a simplified representation of the reality, while slice is a term which better describes reality. Indeed strictly seen, a plane has no thickness while a slice a certain thickness. Nevertheless ideal conditions are considered when the slice thickness tends to be very small, so that the term section plane can be understood as the plane on which the ideal image is desired.
U.S. Pat. No. 4,871,966 discloses a technique known as Multi Slice Multi Angle. According to this method a two dimensional multislice image acquisition is carried out in which single two dimensional images are acquired along different slices or section planes, which section plane or planes may not be parallel or are not parallel one with respect to the other, but the planes have different orientations in space. The method disclosed in the said document allow the generation of images of slices of the anatomic region under examination which have different positions and orientations. The two dimensional images have different positions and orientations relative to only one axis of angular displacement of the said slices or section planes. Taking different images along selected slices having different positions and orientations is advantageous since it is possible to choose the slice orientation in such a way to match different diagnostic aims in order to obtain images of an anatomical region which better bring to evidence a pathological condition if present.
Nevertheless the limitation to the different positions and orientations only relative to only one axis of angular displacement strongly limits the capability of entirely imaging a region. An anatomical object such as for example an intervertebral disk may be oriented along a plane or may define a plane which has a different orientation with respect to the other disks relatively to two different axis of angular displacement. In this case a selected slice which orientation is defined relative to one of the axis of angular displacement could in certain circumstances cut only partially the disk or cut the disk at different depth, not coinciding with a central section plane of the corresponding disk.
If for example one considers to acquire images of axial slices which are positioned at the intervertebral disks in order to maximize the visualization capability of eventually occurring disk protrusions, the known technique would lead to an image of the protrusion only if it will fall on the selected section plane. If however the disk orientation is such that the plane defined by the disk is rotated in space relative to two axes of angular displacement, the protrusion may lay outside the selected image slice or section plane if this slice or section plane is oriented correspondingly to the orientation of the plane defined by the disk only relative to one of the two axis of angular displacement.
According to the known technique, the image slices are chosen having an orientation which is generally substantially parallel or coinciding with the position and orientation of the plane defined by the disk only relative to one axis of angular displacement. In case the spine is not rectilinear in shape according to two different transverse axes relative to the longitudinal extension of the spine, each slice which has to cross a disk must have a corresponding orientation in space relative to the said two axes so each slice must have different angles of orientation with respect to the image slices of other disks relative to the said two different transversal axis and this is equivalent to an angular displacement of the section plane or of the slice according to two different axis of rotation or angular displacement.
The capability of providing image slices of an anatomical region with a different orientation in space relatively to two or three axis of angular displacement or rotation becomes more important if considering anatomical districts having an even more complex structure than the spine.
A further draw back of the known method lies in the fact that a so called scout acquisition is needed in order to define the reference system for selecting the section plane or section planes along which the sequence of the two dimensional images has to be carried out. After having carried out this scout acquisition and having selected the slices an image acquisition sequence is started. Each image along each one of the selected slices is acquired separately and one after the other. If the patient displaces itself between the acquisition steps of the images along two successive image slices, than there is a risk that due to the displacement of the patient the slice along which the image is taken in the second acquisition step does not coincide with the slice image selected in the scout image. Thus the imaging session has to be repeated at least partially for example by repeating at least the scout image acquisition, the selection of the desired image slices and the sequence of the image acquisition steps along the selected image slices. This can be very time consuming if the user of the apparatus realizes that the patient has displaced itself. If however the user does not realize that the patient has changed its position the sequence of images acquired along the selected image slices does not correspond to the desired one and this could results in errors in reading the images and identifying a pathologic condition.